This invention relates to the control of the optical quality of the beam path at the output of high energy lasers, and, in particular, to a device for interfacing two different environments which can occur within the beam path, so that there is a tolerable level of optical distortion arising from transverse variations in refractive index.
The invention also relates in particular to the requirement that occasionally arises in the design of such beam control systems for a device which is capable of propelling a moderate flow of the beam conditioning gas along the axis of an extended length of beam duct, in order to provide a forced convection of the heat absorbed from the beam and turbulent mixing to suppress systematic transverse gradients in refractive index.
The alternative to axial flow, for suppression of transverse gradients, is a forced transverse flow of the beam conditioning gas. However this requires relatively large clearances for the ducting of the required flowing gas, and may not be feasible for some portions of the beam path. A most notable example generally occurs within the relatively confined spaces of a beam directing telescope, in which the beam path must be carried through several gimbal axes.
This invention relates primarily to the device that provides the transition that must be made between the environment of a transversely flowing beam path conditioning gas, and the axially-flowing gas that may be injected into the relatively constrained beam path within the beam directing telescope system. The pressure rise that is introduced by such an axial flow injection system is typically a very small fraction of one atmosphere, and is intended to be sufficient only to overcome the flow losses and to withstand variations in the ambient wind pressure that may be incident on the aperture of the beam director. It is not intended to provide the pressure barrier between the atmosphere and the relatively low pressure in the interior of the laser device itself.
One type of device for separating the two gaseous environments is a solid optical window. One major disadvantage of the solid optical window is the degradation of the window itself by the laser beam. Extrinsic factors such as dirt upon the window may cause the coatings used to disintegrate because of increased temperature, especially that in a high energy laser. Intrinsic factors such as defects in coatings and optical material could also lead to the breakdown and catastrophic failure of the window.
Uneven heating of the solid window material can also result in substantial transverse variations in refractive index, and consequent degradation in optical quality. Limitations on the availability of suitable materials in the required sizes and purity has also inhibited the application of solid windows to high energy laser systems. As a result, the use of aerodynamic windows for high energy lasers has been almost mandatory, to avoid such problems.
An example of a transverse flow supersonic aerodynamic window which is capable of withstanding a relatively substantial pressure difference is shown in U.S. Pat. No. 3,873,939. Therein, a conduit having a flowing gas under pressure is connected to an opening in a passageway that has the laser beam therein. An opposing opening receives the flowing gas such that the laser beam is substantially perpendicular to the flow of the gas across the passageway in the conduit. An array of vanes is placed in the entrance of the conduit to promote a vortex-free flow. The gas in the exit conduit can be either exhausted or recirculated to the pump.
The transverse-flow aerodynamic window utilizes the high lateral momentum of the gas flow to create an effective barrier to allow dissimilar gas types, temperatures, and pressure fields to exist on opposite sides of the gas flow.
These drawbacks have motivated a search for alternative devices that would satisfy the above disadvantages and have additional advantages.